1. Field of the Invention
The present invention relates to a novel delivery system for physical, i.e., inorganic, sunscreens, and the use of said delivery system in the preparation of stable sunscreen compositions.
2. Description of the Related Art
Sunscreen products may be emulsions, creams, lotions, gels, liquids, solid sticks, aerosols and all other forms of cosmetic compositions. Sunscreen compositions are applied topically to human skin to protect the skin against UV radiation damage. The term "sunscreen" herein is meant to include tanning lotions, sunscreens and sunblockers intended for topical application to human skin and/or hair to protect against ultraviolet radiation from the sun.
Active sunscreen ingredients which filter UV-A or UV-B rays harmful to the skin may be organic sunscreens such as PABAs (p-aminobenzoic acids), benzophenones, salicylate esters, and di-oxybenzone, octyl methoxycinnamate, and mixtures thereof, or physical sunscreens, which are inorganic compounds such as titanium dioxide and zinc oxide.
It has been found that inorganic sunscreen agents such as TiO.sub.2 and zinc oxide are superior to customary organic sunscreen agents, as they are less irritating and offer better or broader UV protection. However, inorganic sunscreen agents are difficult to incorporate into sunscreen formulations. The invention relates to a novel delivery system for these inorganic sunscreen agents.
Micronized, also referred to as "microfine", physical sunscreens, such as zinc oxide and titanium dioxide, have a preferred particle size less than 1 micron. These particles of less than 1 micron or submicron size are effective sunblock agents. They do not scatter light, and therefore are useful in cosmetics, make-up, and sunscreen applications. Particles larger than 1 micron are undesirable as they inherently result in so-called "whitening effects".
The problem is that micronized TiO.sub.2 and zinc oxide particles are susceptible to agglomeration after their production, during the post-manufacture period of transportation and handling. This tendency to agglomerate into clumps of much higher particle sizes (greater than 1 micron) reduces the efficacy of TiO.sub.2 and Zinc Oxide as UV sunscreens and increases their white appearance on the skin. The ability of the micronized, inorganic sunscreens to function as UV-ray blocking agents is impeded if the micronized particle size is enlarged by agglomeration or dusting. Agglomeration also results in the deterioration of emulsions including such particles and negatively affects their stability and shelf-life during storage.
Not only are the fine, micronized inorganic sunscreen powders susceptible to agglomeration, but these particulates also have a tendency to show dusting effects during handling, such as charging to the mixing vessels.
The disadvantages of dusting and agglomeration, or re-agglomeration, are avoided by the novel delivery system of this invention.
Numerous references describe the use of micronized zinc oxide and titanium dioxide as sunscreen materials, in various formulations. None of these references, however, teach or suggest the specific novel delivery system of physical sunscreens of this invention, which prevents inorganic sunscreen pigment particles from accumulating to give agglomerates.
U.S. Pat. No. 4,323,693 to Scala, Jr. discloses a benzoic acid ester of isostearyl (C18) alcohol.
U.S. Pat. No. 4,917,882 to Stobridge discloses a gel-type sunscreen composition and a method of making same, comprising combining a sunscreen agent, polyethylene, and a benzoic ester, agitating and heating the mixture to a temperature and for a time sufficient to dissolve the polyethylene in the benzoate ester, and after the polyethylene is dissolved in the benzoate ester, cooling the mixture while agitating to produce a gelled sunscreen composition.
U.S. Pat. No. 5,340,567 to Cole et al. discloses a sunscreen composition comprising an extending medium such as a carrier or vehicle (such as mineral oil and polyethylene) and a synergistic combination of microfine titanium dioxide having a particle size of less than about 35 mu and zinc oxide having a particle size of less than about 50 mu. The particle sizes of the titanium oxide and zinc oxide are critical for obtaining a composition which is invisible on the skin.
U.S. Pat. No. 5,417,961 to Nearn et al. discloses a sunscreen composition comprising a water-in-oil emulsion which comprises an aqueous phase and an oil phase, the oil phase comprising polyethylene and an organic sunscreen agent. Microfine zinc oxide having a particle size in the range of from about 0.01 microns to about 0.25 microns is suspended in the oil phase.
U.S. Pat. No. 5,468,471 to Zecchino et al. discloses an organic dispersion of microfine titanium dioxide, of a particle size prior to agglomeration of about 10 nm to about 100 nm, and a cosmetically acceptable branched chained organic compound, preferably octyldodecyl neopentanoate, without any dispersing agent. The dispersion is prepared by subjecting the microfine titanium dioxide and the branched chain organic compound to a ball mill, roller mill or ultrasonic mixer, to grind the titanium dioxide in the organic compound and to disperse the titanium dioxide in the organic compound.
U.S. Pat. No. 5,476,643 to Fogel discloses use of two neopentyl glycol diesters as wetting, dispersing, spreading and detergent agents for micronized titanium dioxide and zinc oxide.
U.S. Pat. No. 5,498,406 to Nearn et al. discloses a sunscreen composition in oil-in-water emulsion form having about 0.5% to about 5% by weight of microfine titanium dioxide having a particle size of less than about 100 nm uniformly suspended therefrom, the composition further comprising a dispersing agent comprising a long chain saturated primary alcohol having an average of from about 25 to about 45 carbon atoms in the long chain, to stabilize the emulsion.
U.S. Pat. No. 5,543,136 to Aldous discloses making and using a water-in-oil emulsion comprising zinc oxide and an agent selected from the group consisting of tridecyl neopentanoate, C.sub.12-15 alkyl benzoate, octyl neopentanoate and mixtures thereof in the emulsion's oil phase; titanium dioxide in the emulsion's water phase, a sunblocking agent, and oil phase emulsion components.
U.S. Pat. No. 5,605,652 to Tapley discloses a method of preparing sunscreens in which a dispersion of zinc oxide particles in an oil is formed by milling in the presence of a particulate grinding medium and mixed with cosmetically acceptable materials. A mixed oxide dispersion comprising an oil, particles of zinc oxide, particles of titanium oxide, and an organic dispersing agent is formed.
U.S. Pat. No. 5,573,753 to Tapley discloses a method of preparing sunscreen containing a zinc oxide dispersion comprising milling a particulate zinc oxide in an oil in the presence of a particulate grinding medium and an organic dispersing agent.
U.S. Pat. No. 5,599,529 to Cowie discloses an oil dispersion comprising an oil, titanium dioxide particles, and an organic dispersing agent for the particles. The dispersion has a solids content of greater than 40% by weight. The oil is selected from the class of oils consisting of fatty acid esters, fatty alcohols and saturated fatty acid di-esters. The oil dispersion is prepared by milling particulate titanium dioxide in an oil in the presence of a particulate grinding medium and an organic dispersing agent.
U.S. Pat. No. 5,725,844 to Gers-Berlag et al. discloses a sunscreen in the form of an oil in water emulsion or a hydro-dispersion, comprising one or more silanized hydrophobic inorganic pigments incorporated into the oily phase of the emulsions or hydrodispersions, one or more oil-soluble UV filter substances, one or more film-forming agents and optionally one or more water-soluble UV filter substances.
U.S. Pat. No. 5,730,993 to Allard et al. discloses a sunscreen comprising an ultrafine oil-in-water emulsion resulting from phase inversion of homogeneously and finely dispersed particulate of at least one nanopigment comprising a metal oxide, such as titanium dioxide or zinc oxide, having a particle size from 100 nm to 1,000 nm.
U.S. Pat. No. 5,747,012 to Dahms discloses a process for preparing a sunscreen comprising mixing a dispersion in an oil of particles of metallic oxide having an average particle size of less than 0.2 micrometers, with one or more emulsifiers, an aqueous phase, and a hydrophilic organic sunscreen, under conditions in which an emulsion is formed. The emulsion may be an oil-in-water emulsion or a water-in-oil emulsion. The dispersion of particles of metallic oxide is prepared by milling the particulate metallic oxide in the oil in the presence of a particulate grinding medium and in the presence of a dispersing agent.
U.S. Pat. No. 5,776,440 to Forestier et al. discloses a sunscreen composition comprising at least one coated nanopigment of metallic oxides, and at least one UV-screening fat-soluble polymer, in a cosmetically acceptable carrier.
U.S. Pat. No. 5,788,952 to Gers-Berlag et al. discloses a sunscreen comprising inorganic micropigments and optionally additional organic UV filter substances. The formulation is a hydrodispersion consisting of an inner lipid and an outer aqueous phase, free of emulsifiers, with the organic micropigments incorporated in the preferably liquid lipid phase of the hydrodispersion.
U.S. Pat. No. 5,817,298 to Galley et al. discloses a sunscreen composition comprising a water-in-oil emulsion which comprises 0.5 to 50% by weight of titanium dioxide particles having a mean particle size of less than 100 nm, each of the particles being substantially coated with phospholipid, together with a cosmetically acceptable carrier. Coating titanium dioxide particles with phospholipid reduces their tendency to clump.
The references describe various methods to prevent or reduce clumping of micronized physical (inorganic) sunscreens, so as to improve the efficacy of the sunscreen composition. However, among the foregoing patents, none disclose the specific delivery system for physical (inorganic) sunscreens of the invention or the use of such systems in the production of stable sunscreen compositions.
Thus, none of the references appear to teach or suggest a delivery system for physical (inorganic) sunscreens in which micronized sunscreens are embedded in a solid product which does not require mixing or regrinding before use. A solid formulation avoids the problems inherent in a fluid emulsion or dispersion, i.e., migration or settling out of the contents, which requires mixing and/or regrinding before use.